METHOD FOR TREATING A PART MADE OF IRON ALLOY FOR IMPROVING THE ANTI-CORROSION PROPERTIES THEREOF

Abstract

The present invention relates to a method for treating a part (P) made of iron alloy for improving the anti-corrosion and mechanical strength properties thereof, the method comprising: a salt bath nitriding or salt bath nitrocarburising step, to form a combination layer (1) on the part (P), and subsequently a step of phosphating the part (P), to form a phosphating layer (2) on the surface of the part,
characterised in that the bath of molten salts contains chlorides, and the phosphating step is carried out in a phosphating bath which contains zinc ions and/or manganese ions, and iron ions.

Claims

1-13. (canceled)

14. A method of increasing anti-corrosion and mechanical strength of an iron alloy part, the method comprising: forming a combination layer on the iron alloy part by using a nitriding molten salt bath and a nitrocarburising molten salt bath, wherein the nitriding molten salt bath and the nitrocarburising molten salt bath comprise chlorides; and phosphating the iron alloy part to form a phosphating layer on a surface of the iron alloy part, wherein the phosphating is carried out in a phosphating bath comprising zinc ions and/or manganese ions, and iron ions.

15. The method according to claim 14, wherein the molten salt bath does not contain sulphur.

16. The method according to claim 14, wherein the chlorides comprise alkaline metal chlorides.

17. The method according to claim 16, wherein the alkaline metal chlorides are lithium, sodium, potassium chlorides, or a combination thereof.

18. The method according to claim 14, wherein the molten salt bath comprises: 25% w/w to 60% w/w of alkaline metal chlorides; 10% w/w to 40% w/w of alkaline metal carbonates; 20% w/w to 50% w/w of alkaline metal cyanates; and 3% w/w or less of cyanide ions.

19. The method according to claim 14, wherein the iron ions are present in the phosphating bath at a mass concentration of between 1 g/l and 8 g/l, and wherein: the phosphating bath contains zinc ions but no manganese ions, and said zinc ions are present at a mass concentration of between 1 g/l and 40 g/l with respect to the total volume of the phosphating bath; the phosphating bath contains manganese ions but no zinc ions, and said manganese ions are present at a mass concentration of between 1 g/l and 40 g/l with respect to the total volume of the phosphating bath; or the phosphating bath contains zinc ions and manganese ions, and the total of these ions is present at a mass concentration of between 1 g/l and 40 g/l.

20. The method according to claim 19, wherein the phosphating bath contains zinc ions but no manganese ions, and said zinc ions are present at a mass concentration of between 5 g/l and 20 g/l with respect to the total volume of the phosphating bath.

21. The method according to claim 19, wherein the phosphating bath contains manganese ions but no zinc ions, and said manganese ions are present at a mass concentration of between 5 g/l and 20 g/l with respect to the total volume of the phosphating bath.

22. The method according to claim 19, wherein the phosphating bath contains zinc ions and manganese ions, and the total of these ions is present at a mass concentration of between 5 g/l and 20 g/l.

23. The method according to claim 19, wherein the iron ions are present in the phosphating bath at a mass concentration of between 1 g/l and 6 g/l.

24. The method according to claim 14, wherein the phosphating layer has a thickness of between 3 m and 40 m.

25. The method according to claim 24, wherein the phosphating layer has a thickness of between 5 m and 30 m.

26. The method according to claim 25, wherein the phosphating layer has a thickness of between 5 m and 20 m.

27. The method according to claim 14, wherein the combination layer has a thickness of between 5 m and 40 m.

28. The method according to claim 27, wherein the combination layer has a thickness of between 15 m and 25 m.

29. The method according to claim 14, wherein the iron alloy part is made of grey cast iron.

30. A part made of grey cast iron, comprising: a nitriding layer in contact with the grey cast iron, comprising a combination layer containing nitrides; and a phosphating layer arranged on and in contact with the combination layer, comprising metal phosphates, wherein the metal phosphates comprise zinc, manganese, or a combination thereof, and iron.

31. The part according to claim 30, wherein the phosphating layer has a thickness of between 3 m and 40 m.

32. The part according to claim 30, wherein said part comprises graphite inclusions at a surface of the part on which the combination layer is formed.

33. The part according to claim 30, wherein the part comprises a brake disc.

Description

DESCRIPTION OF THE FIGURES

[0071] Other advantages and features of the invention will appear upon reading the following description given as a non-limiting, illustrative example, in reference to the following accompanying figures:

[0072] FIG. 1 is a micrograph of a part according to an embodiment of the invention, said part being a brake disc made of lamellar cast iron.

[0073] FIG. 2 is a photograph of the part of FIG. 1 after nitriding/phosphating treatment and before a saline spray test.

[0074] FIG. 3 is a photograph of a part not according to the invention, said part being a brake disc made of lamellar cast iron.

[0075] FIG. 4 is a photograph of the part of FIG. 3 after nitriding/phosphating treatment and before a saline spray test.

Detailed Description of Embodiments of the Invention

[0076] The approach of the Applicant has been to carry out several series of tests implementing different treatments of a part (P) made of grey cast iron.

[0077] The problem addressed consists of finding a method conferring a sufficient resistance to corrosion on the part (P), despite the fact that the nitriding bath does not contain sulphur and that in addition, said part is made of grey cast iron which is non-degraphitised, and therefore particularly difficult to protect from corrosion.

[0078] The treated parts (P) have been subjected to a saline spray, and their resistance to corrosion is evaluated from the duration of appearance of a pitting and/or rust run-off threshold.

[0079] Then, the second problem has been to validate the correct operation of the part (P) thus treated, according to a particular use of the brake disc: comparative friction coefficient measurements have therefore been taken.

[0080] In particular, the Applicant has studied the effects of the two following steps.

[0081] In reference to the micrograph illustrated in FIG. 1, the ARCOR nitriding treatment (trademark filed by the Applicant) provides, from the surface to the part (P) core, a juxtaposed combination layer (1) and diffusion zone (1). The combination layer (1) typically has a thickness of between 5 and 30 m, and is formed at the surface of the substrate(S) which composes the part (P).

[0082] The diffusion zone (1) typically has a thickness of a few tens or hundreds of microns, for example, 300 m, and is measured from the surface of the substrate(S) of the part (P), in the direction of the core of said part.

[0083] Nitriding has been carried out in a bath of molten salts.

[0084] Phosphating provides, on the surface of the part (P), above the combination layer (1), a metal phosphate layer (2). The metal phosphate layer (2) typically has a thickness of around 20 m.

[0085] On the micrograph of FIG. 1, an additional layer (a) can be seen: this is an aluminium layer not forming part of the part (P), but deposited in view of performing the cutting necessary for the micrograph.

[0086] Furthermore, the lamellar-shaped graphite inclusions (G) can be observed, which open on the surface of the part (P).

[0087] A first series of anti-corrosion tests has been carried out. The parts (P) manufactured as grey cast iron-based, and subsequently treated, are placed in an enclosure and subjected to a saline spray test.

[0088] The enclosure is then opened every 24 hours, in order to verify the appearance of corrosion points, which provide evidence of a start of localised corrosion. These points are counted, and if there is more of them than a predefined threshold, for example 50, then the part (P) is considered as corroded and the test is stopped.

[0089] The longer the duration is before the threshold is reached, the more resistant the part is to corrosion. A minimum resistance to corrosion is fixed by the Applicant to a duration greater than or equal to 96 hours.

[0090] The table 1 below illustrates the corrosion tests which are more relevant for understanding the invention:

TABLE-US-00001 TABLE 1 Nitriding Nitriding Post- Duration before No. Pretreatment media salt treatment corrosion threshold Result C4 Without Liquid ARCOR V Oxidation 1 24 hours NG C5 Without Liquid ARCOR L Without 24 h NG C8 Without Liquid ARCOR L Oxidation 2 24 h NG C9 Without Liquid ARCOR L Oxidation 1 48 h NG C10 = P Without Liquid ARCOR L Ph (ZnFe) >96 h OK C13 Without Liquid ARCOR L Ph (ZnCa) 24 h NG C14 Without Liquid ARCOR L Ph (Zn) 24 h NG C16 Without Liquid ARCOR V Ph (ZnFe) 24 h NG C17 Without Liquid ARCOR V Ph (ZnCa) 24 h NG

[0091] An ARCOR L nitriding is a ferritic nitrocarburising method with treatment temperatures being able to vary between 530 C. and 650 C. according to the steel to be treated and to the requested specification. It corresponds to what is described in document FR2972459. The ARCOR L nitriding bath contains chloride ions.

[0092] An ARCOR V nitriding is a ferritic nitrocarburising method with treatment temperatures being able to vary between 500 C. and 700 C. according to the steel to be treated and to the requested specification. It corresponds to the nitriding part which is described in document FR2812888. The ARCOR V nitriding bath does not contain chloride ions.

[0093] An Oxidation 1 post-treatment is a method for oxidising in a bath of molten salts at a temperature of 450 C.

[0094] An Oxidation 2 post-treatment is a method for oxidising in an aqueous solution with treatment temperatures being able to vary between 120 C. and 140 C. The salts are mainly formed of nitrates, nitrites and carbonates associated with Na+ alkaline cations.

[0095] A Ph (ZnFe) post-treatment is a method for phosphating with treatment temperatures being able to vary between 60 C. and 70 C. The salts used are of the (H.sub.2PO.sub.4).sub.2Me type and comprise Zn.sup.2+ ions (Me=Zn). Iron is provided in iron powder form.

[0096] A Ph (ZnCa) post-treatment is a method for phosphating with treatment temperatures being able to vary between 80 C. and 90 C. The salts used are of the (H.sub.2PO.sub.4).sub.2Me type and comprise Zn.sup.2+ et Ca.sup.2+ ions (Me=Zn, Ca).

[0097] A Ph (Zn) post-treatment is a method for phosphating with treatment temperatures being able to vary between 60 C. and 70 C. The salts used are of the (H.sub.2PO.sub.4).sub.2Me type and comprise solely Zn.sup.2+ ions.

[0098] Based on the corrosion tests carried out, it is possible to draw the following conclusions.

[0099] In reference to the photograph of FIG. 2, it is seen that a part (P) according to the invention has no corrosion point (photo taken at the end of 48 hours of testing).

[0100] In reference to the photograph of FIG. 3, a part (C14) not corresponding to the invention has, at the end of 48 hours of testing, more than 50 corrosion points (pc). The treatment referenced C14 is therefore not satisfactory.

[0101] In reference to FIG. 4, a micrograph of a part (C14) has been carried out after treatment, and before the saline spray test. It is observed that the boundary between the phosphating layer (2) and the combination layer (1) is less clear than on the parts (P) according to the invention. It can be assumed that the deposition of the phosphating layer (2) has attacked the combination layer (1), insofar as phosphating with Zn alone removes Fe ions in the substrate. Contrary to substrates made of zinc-phosphated iron alloy, but not having undergone nitriding, it is observed that phosphating with zinc alone on nitrided parts does not give a good corrosion protection. A theory with this observation is that the dissolution of iron from the combination layer makes the surface of the substrate(S) too rough, which could increase the porosity of the phosphate layer.

[0102] This phenomenon is not observed when the phosphating bath comprises Zn.sup.2+ ions and Fe.sup.2+ ions (test C10).

[0103] From all the different nitriding and phosphating/oxidising combinations tests on the parts (P) not having undergone degraphitising, only the combination of an ARCOR L nitriding and a Ph (Zn and/or Mn)+Fe) phosphating makes it possible to obtain a sufficient resistance to corrosion (test C10).

[0104] Other tests have then been carried out in order to verify that the features of the treated part (P) are compatible with a use as a brake disc.

[0105] It is observed that the presence of manganese ions in the phosphating bath confers a lower friction coefficient to the part (P), which is not compatible with a use as a brake disc.

[0106] However, a part (P) not having undergone degraphitising, and having undergone an ARCOR L nitriding and a Ph (ZnFe) phosphating is absolutely compatible with such a use.

[0107] In this regard, the friction performance of such a part (P) is close to that of the prior art. The method according to the invention is therefore particularly adapted to the treatment of brake discs.

[0108] The method according to the invention is however less expensive than the solutions of the prior art, as it makes it possible to avoid degraphitising. The nitriding step, in particular, is inexpensive, as the nitriding salts used (chlorides) are economical and their dosage is not critical.

[0109] In other words, the method according to the invention only requires a nitriding and a phosphating to modify the chemistry and the macroscopic structure of the part (P), and thus confer it good mechanical properties, in particular of mechanical strength, resistance to wear, and resistance to corrosion.

[0110] In addition, with nitriding and phosphating both being carried out by bath, there is no need to change the mounting of the parts to move from one step to another, and industrialisation is facilitated. In particular, the time for carrying out the complete treatment is broadly reduced with respect to an equivalent gaseous nitriding.

[0111] Other ferrous alloys can be considered to produce the part (P), such as steel or white cast iron. However, grey cast iron has the interest of being the least expensive material.